Photochromic naphthopyran compositions

ABSTRACT

Photochromic naphthopyran compositions containing minor amounts of either a base or a weak-to-moderate acid are disclosed. Addition of the acid or base increases the fading rate of the colored naphthopyran compositions making them useful in eye protection applications such as sunglasses.

United States Patent 72] Inventors Joseph Casella Framingham;

Samuel H. Stein, Lexington, both of Mass. 862,965

Oct. 1, 1969 Dec. 14, 197 l Itek Corporation Lexington, Mass.

[21] Appl. No. [22] Filed [45] Patented [73] Assignee [54] PHOTOCHROMICNAPHTHOPYRAN [50] Field of Search [56] References Cited UNITED STATESPATENTS 3,451,81 l

6/1969 Brynko 96/1 3,441,41 l 4/1969 Amidon et a1. 96/90 PC 3,361,706H1968 Meriwether.... 96/90 PC 3,341,330 9/1967 Foris 96/90 PC 3,299,0791/1967 Taylor 96/90 PC OTHER REFERENCES Journal of Chemical Society(England) Cottam et a1. 1964 Dec. pp. 5,228- 5231 Becker et al. Journalof Physical Chemistry, Vol. 72 (3) pages 997- 1000 1968) PrimaryExaminer-George F. Lesmes Assistant Examiner-J. P. BrammerA!torneysHomer 0. Blair, Robert L. Nathans, W. Gary Goodson and David E.Brook ABSTRACT: Photochromic naphthopyran compositions containing minoramounts of either a base or a weak-to-moderate acid are disclosed.Addition of the acid or base increases the fading rate of the colorednaphthopyran compositions making them useful in eye protectionapplications such as sunglasses.

PHO'I'OCHROMIC NAPI'ITHOPYRAN COMPOSITIONS BACKGROUND OF THE INVENTION 1Field of the Invention This invention relates to photochromiccompositions and more particularly to photochromic naphthopyrancompositions and a method for increasing the rate of fading ofirradiated naphthopyran species.

2. Description of the Prior Art Photochromism is that property of achemical system wherein it exhibits a reversible change in its visibleabsorption spectrum upon exposure to and removal of activatingwavelengths of light. Consequently, the photochromic system will changefrom its original color to a second color upon itradiation withactivating light. When the irradiation is removed, the system willrevert to its original color. For a comprehensive discussion ofphotochromic systems, see the article entitled Photochromism" byDessauer & Paris which appears at pages 275-317 of Noyes, W. A., HammondG. S., and Pitts, J. N., Advances in Photochemistry, vol. I, I963).

One problem with many of the existing photochromic compositions is thesubstantial difference between the rate at which they change from theiroriginal color to the irradiated color and the rate at which they revertto the original color when the irradiation is removed. While mostphotochromic compounds change to their colored species rapidly uponbeing irradiated, there is a great variance in the rate at which thesecompounds revert to their original species. Some compounds revert almostinstantaneously, while others remain in the irradiated colored state forhours, days, months or even years, depending upon their molecularstructure.

The desired rate for reversion depends, of course, on the ultimate usefor the particular photochromic. For example, a photochromic systemwhich reverts to the original state rapidly cannot be successfully usedin data storage applications unless a method is found to slow the rateof reversion (i.e., iix") of the photochromic composition from itsirradiated state. One method for overcoming this problem withphotochromic benzo-indolinospiropyrans is to subject the colored speciesof these photochromics to hydrohalic acid vapors (HCl, HBr, etc.) SeeForis, US. Pat. No. 3,341,330. The effect of the hydrohalic vapors canbe neutralized by subsequently contacting the treated photochromicspiropyran material with alkaline vapors.

For other applications of photochromic systems, the problem is exactlyopposite to that encountered with attempts to use spiropyrans for datastorage. Naphthopyrans, for example. typically have slower reversionrates which make them unsuitable for use in eye protection devices suchas sunglasses wherein rapid reversion rates are required. Typicalnaphthopyrans have reversion rates which range from several hours tomany days to reach complete reversion.

One method that has been used to increase the reversion rate ofphotochromic naphthopyrans is the application of heat to the irradiatedspecies. Heating is often unsatisfactory, however, for many reasons. Itis usually inconvenient to use heating each time a reversion occurs andheating normally requires special equipment which is cumbersome and addsexpense to any photochromic device. Even with heating, typicalnaphthopyrans have reversion rates of several hours or more when heatedto 50-60 C. Additionally there is a limit on the temperature to whichphotochromic naphthopyrans can be heated before they begin to experiencethermal degradation.

SUMMARY OF THE INVENTION It has now been found that the fading rate ofphotochromic naphthopyran compositions can be surprisingly increased byadding minor amounts of either a base or a weak to moderate acid to thenaphthopyran compositions. Because of the marked increase in fadingrates, the naphthopyran compositions which can be obtained with thisinvention, can be successfully used in eye protection devices andsimilar applications where rapid reversion upon removal of theirradiation is desirable.

DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention is useful withnaphthopyrans which exhibit photochromism. In their nonirradiated state,these naphthopyrans have the following structural formula and numberingsequence:

R are substituents individually selected from R,- substituents; pors-amine, and amide.

It is believed that these naphthopyrans exhibit photochromism whenirradiated with ultraviolet light because the pyran ring structure iscleaved between the oxygen and number two carbon atom. Thus, thephotochromic reaction, in tenns of structural formulas, appears to be:

R1 R2 R 0 CH 0 1 CH U.V. CH R2 R4 Rs R4 R3 (irradiated) (nonirradlated)Some specific examples of photochromic naphthopyrans useful with thisinvention include, but are not limited to, the naphthopyrans representedby the following structural formulas in their nonirradiated state:

2,2-dlphenyl-2H-naptho(l,2-b)pyran (l /C) O CH2,2-(11(4-chlorophenyl)-2H-naphtho(1,2-b)pyran2pheriyl-2-(4-meth0xypyenyl)-2H-naphtho(l,2-b)pyran2,2-d1methyl-6,8-dlmsthoxy-2H-uaphtho(1,2-b) pyran Photochromicnaphthopyrans can be used by forming dispersions of the naphthopyran inpolymeric binders or plastisols, or by dissolving them in solvents. Ingeneral, polymers such as polymethyl methacrylate, polystyrenes, etc.,are suitable as binders and organic solvents such as benzene,chloroform, methyl ethyl ketone, ethanol, etc., are suitable to dissolvethe naphthopyrans. Those skilled in the art will know or easily be ableto determine appropriate binders and solvents by routineexperimentation.

Minor amounts of bases or weak to moderate acids are used to increasethe fading rate of the above-described photochromic naphthopyrans. Theexact amount of base or acid depends on such factors as the particularnaphthopyran composition, the amount of photochromic material present,

the particular acid or base used, the strength of the acid or.

base used, and the desired fading rate of the photochromic material.Using these and other factors, those skilled in the art will be able todetermine the exact amount of a particular base or acid to be used withspecific photochromic naphthopyran compositions. Preferably, an amountof acid or base below about percent by weight based on the totalphotochromic composition will be used and an amount of below about 5percent is particularly preferred. Often, amounts as low as 0.1 percentare effective.

It has been found that acids having weak to moderate strengths can besuccessfully used to increase the fading rate of photochromicnaphthopyran compositions, but strong acids do not have this effect. Acommon method for expressing the relative strengths of acids is in termsof pKa values, which are equivalent to the negative logarithm of anacids dissociation constant in water. Using the pKa values given atpages D--90-l of the CRC Handbook of Chemistry and Physics, 49thEdition,

1968-1969, it has been found that acids having pKa values between about4 and l4 can be effectively used to control the fading rate ofphotochromic naphthopyrans. The preferred acids are those having pKavalues of from about 4.5 to about 9.

Within the aforementioned strength limitations, all types of acids canbe used. Both inorganic and organic acids increase the fading rate ofphotochromic naphthopyrans. Additionally, organic acids can be monoorpolycarboxylic and can be saturated or unsaturated. Organic carboxylicacids are particularly advantageous for certain photochromicapplications such as sunglasses because they are nonvolatile and caneasily be incorporated in polymeric binders.

While only weak to moderate acids can be used, it has been found thatbases will operate whether they are strong, moderate or weak instrength. Similarly to the acids, a common method of expressing thestrength of bases is in terms of pKb values which are equal to thenegative logarithm of a bases dissociation constant in water. The pKbvalues can be calculated from the pKa values given at pages D-88-9 ofthe CRC Handbook of Chemistry and Physics using the formula pKa+ pKb=l4.Using pKb values, it has been found that bases having a pKb value offrom about 2 to about 14 can be used to control the fading rate ofphotochromic naphthopyrans. The preferred bases have pKb values of fromabout 2.5 to about 9.

Within the strength limitations given, both inorganic and organiccompounds which contain one or more basic functional groups can be used.

For polyfunctional acids or bases, the respective pKa and pKb valuesthat should be within the indicated ranges are those values for thefirst step in the dissociation process. For example, sulfurous acid hasa pKa value of about 1.81 for step 1, and therefore would not be in theoperative range of pKa values from 4-14 even though its pKa value forstep 2 is about 6.91.

The pKa and pKb values used in determining operative and preferred baseand acid strengths should be taken at about 25 C.

The mechanism involved in controlling the fading rate with minor amountsof bases or moderate to weak acids is not thoroughly understood. Itappears, however, that a different mechanism is involved for acids thanis involved for bases. The following explanations are not intended to bebinding, but are only offered as possible explanations of thesephenomena.

With acids, a possible explanation is that the irradiated form makescataylic use of a proton supplied by the acid in its rearrangement tothe nonirradiated state. Expressed in terms of structural formulas, thiswould be as follows:

Support for this explanation is found in the fact that the solutions ofphotochromic naphthopyrans retain their photochromic properties evenafter addition of the acid which indicates that no pennanent additionsare made to the molecules by the acids.

With bases, the site of the reaction is more likely to be the 21-!carbon atom of the pyran ring, which is electrophilic and subject tonucleophilic attack. In terms of structural formulas and using thenitrogen atom N: which could be obtained from a compound such aspyridine as the base, this would be represented as:

The photochromic naphthopyran compositions and the process forcontrolling the fading rate of such compositions are generally useful inany application wherein photochromic compounds or compositions arepresently used. Some applications for such photochromics which have beendescribed in the literature include photographic dodging, photochromicwindows, photochromic nonglare mirrors and photochromic eye protectiondevices. The compositions and process of this invention are particularlyuseful in producing photochromic sunglasses which are colorless in theabsence of bright sunlight and which are converted to the coloredspecies by the ul traviolet light present in sunlight thereby reducingglare.

Usually the irradiated forms of naphthopyrans are colored and thenonirradiated forms are colorless or very pale colors. This is notalways true, however. Therefore, the term fading rate is used to meanthe rate at which a naphthopyran returns to its nonirradiated form afterirradiation ceases, whether the nonirradiated fonn is colorless or has amore intense color than the irradiated form.

The invention is further illustrated by the following examples. Allparts and percentages are by weight unless otherwise specified.

EXAMPLE I Increased Fading Rate for 2,2-diphenyl- ZH-naphtho l, 2-b)pyran by Adding Acids Techniques for preparing 2,22l-l-naphtho (l, 2-b)pyran are described by Livingstone and Cottam at J. Chem. Soc, p. 5,228,1964. This photochromic naphthopyran has a very pale yellow color(almost colorless) in its nonirradiated state which changes to orangewhen the compound is irradiated with ultraviolet light.

One cell of a Beckman D.U. spectrophotometer was filled with a 7.5Xl0molar solution of 2,22H-naphtho (l, 2-b) pyran in methanol. The othercell of the spectrophotometer was filled with pure methanol. Both cellswere thermostated at 25 C. 0.l C. The initial transmittance beforeirradiation was measured using light with a wavelength of 460 nanometersand found to be 69.5 percent.

Each of the solutions was removed from the spectrophotometer andilluminated with an 8 watt Blacklite" fluorescent tube for 2.5 minutesand then placed back into their respective cells. A first reading wastaken immediately of the transmittance and the time was designated aszero minutes. Periodic transmittance measurements were taken as theorange colored photochromic compound returned to its original form. Thisdata was plotted and used as a control.

The same experiment was repeated twice, except that a small amount ofacetic acid (pKa=4.75 at 25 C.) was added to the photochromicnaphthopyran solution in the first case and a small amount of phenol(pKa=9.89 at 20 C.) was added in the second case. in the case of theacetic acid, approximately one drop was added per 3 mililiters ofsolution. In the case of the phenol, approximately one tiny granule ofthe solid phenol was added per 3 mililiters of solution. The dataobtained was used to form a plot of the percent transmittance versustime.

Results of this experiment taken from the plot were:

% TRANSMIT'I'ANCE AT 460 NANOMETERS As can be seen, the addition ofminor amounts of weak to moderate acids greatly increased the fadingrate of 2,2- diphenyl-ZH-naphtho l, 2-b) pyran.

EXAMPLE ll Increased Fading Rate for 2,2-diphenyl-6-nitro-2H-naphtho 1,2-b) pyran by Adding Acids or Bases 2,2-diphenyl-6-nitro-2H-naphtho (l,2-b) pyran can be prepared by nitrating the six carbon atom of thenaphthalene ring of the compound of example I using glacial acetic acidand fuming nitric acid at room temperature. This is a standard nitrationmethod well known to those skilled in the art. In the nonirradiatedcondition, this photochromic naphthopyran has a pale yellow color(almost colorless) which changes to a reddish orange color uponillumination with ultraviolet light.

The procedure of example I was repeated except that a 31 X l 9: molarsolution of 2,2-diphenyl-6-nitro-ZH-naphtho- (l, 2-b) pyran in benzenewas used. Additionally, the base pyridine (pKa=8.75 at 25 C.) was usedin place of the acid phenol to increase fading rate. Initially, thetransmittance of the photochromic solution was 96 percent measured withlight having a wavelength of 490 nanometers.

Results were:

TRANSMITTANCE AT 490 NANOMETERS photochromic solution.

OH (in wherein: 1

R are substituents individually selected from hydrogen; halide; C,Calkyl; C -C alkoxy; t-amine; phenyl; and substituted phenyl having up totwo substituents individually selected from halide, C C. alkyl, C -Calkoxy, nitro, amide and p-, s, or t-amine; and,

R are substituents individually selected from R substituents; pt ors-amine; and amide; and,

b. from about 0.1 to about percent of either a base of a weak tomoderate strength acid having a pKa of from about 4 to about l4.

2. A composition of claim 1 wherein R and R, represent phenylsubstituents.

3. A composition of claim 2 containing a minor of a base having a pKbvalve measured at 25 C. in the range of from about 2.5 to about 9.

4. A composition of claim 3 wherein said base comprises pyridine.

5. A composition of claim 2 containing a minor amount of an acid havingpKa value measured at 25 C. in the range of from about 4.5 to about 9.

6. A composition of claim 5 wherein said acid comprises acetic acid.

7. A process for increasing the fading rate of the irradiated species ofphotochromic naphthopyrans represented in the nonirradiated form by thestructural formula (6H CH wherein:

R,., are substituents individually selected from hydrogen; halide; C Calkyl; C -C alkoxy; t-amine; phenyl; and substituted phenyl having up totwo substituents individually selected from halide. C C alkyl. -(2ulkoxy. nitro. amide. and p-. sor t-uminc: and

R are aubstituents individually selected from R aubstituents; pors-amines;

and, amide; said process comprising:

contacting the irradiated naphthopyran with a minor amount of either abase or a weak to moderate strength acid.

8. A process of claim 7 wherein R and R, represent phenyl substituents.

9. A process of claim 7 wherein said base has a pKb value measured at 25C. in the range of from about 2 to about 14 and said acid has a pKavalue measured at 25" C. in the range of from about 4 to about 14.

10. a process of claim 7 wherein said irradiated naphthopyran iscontacted with a minor amount of a base having a pKb value measured at25C. in the range from about 2.5 to about 9.

11. a process of claim 10 wherein said base comprises pyridine.

12. A process of claim 11 wherein R and R, represent phenylsubstituents.

13. A process of claim 7 wherein said irradiated naphthopyran iscontacted with a minor amount of an acid having a pKa value measured at25 C. in the range of from about 4.5 to 9.

14. A process of claim 13 wherein said acid comprises acetic acid.

15. A process of claim 14 wherein R and R, represent phenylsubstituents.

2. A composition of claim 1 wherein R1 and R2 represent phenylsubstituents.
 3. A composition of claim 2 containing a minor of a basehaving a pKb valve measured at 25* C. in the range of from about 2.5 toabout
 9. 4. A composition of claim 3 wherein said base comprisespyridine.
 5. A composition of claim 2 containing a minor amount of anacid having a pKa value measured at 25* C. in the range of from about4.5 to about
 9. 6. A composition of claim 5 wherein said acid comprisesacetic acid.
 7. A process for increasing the fading rate of theirradiated species of photochromic naphthopyrans represented in thenonirradiated form by the structural formula
 8. A process of claim 7wherein R1 and R2 represent phenyl substituents.
 9. A process of claim 7wherein said base has a pKb value measured at 25* C. in the range offrom about 2 to about 14 and said acid has a pKa value measured at 25*C. in the range of from about 4 to about
 14. 10. a process of claim 7wherein said irradiated naphthopyran is contacted with a minor amount ofa base having a pKb value measured at 25* C. in the range of from about2.5 to about
 9. 11. A process of claim 10 wherein said base comprisespyridine.
 12. A process of claim 11 wherein R1 and R2 represent phenylsubstituents.
 13. A process of claim 7 wherein said irradiatednaphthopyran is contacted with a minor amount of an acid having a pKavalue measured at 25* C. in the range of from about 4.5 to
 9. 14. Aprocess of claim 13 wherein said acid comprises acetic acid.
 14. 15. Aprocess of claim 14 wherein R1 and R2 represent phenyl substituents.